Lightly spray a 9 x 13 baking pan with cooking spray. Evenly distribute the bread cubes in it to form a big layer. Whisk the eggs, milk, pumpkin puree, pumpkin butter, pumpkin pie spice and salt together in a bowl until smooth. Pour the mixture over the bread. Make sure the bread is in an even layer and covered in the liquid. Cover the baking pan and let it sit in the refrigerator overnight.

The next morning, pre-heat the oven to 350. Take the baking pan out and remove the cover. Bake the casserole for about 45 to 50 minutes. The bread should get slightly crispy on top and the custard should be completely set. Allow it to cool when it's done for about 5 minutes. Then cut into the desired portions and serve immediately with chopped walnuts and syrup on top!